In the manufacture of polyurethane foams, a polyol, an isocyanate, water and at least one catalyst for catalyzing the formation of polyurethane foam are mixed to form a liquid mixture, which then foams and solidifies to produce the solid polyurethane foam. Reaction between the isocyanate and the water produces carbon dioxide gas, this production of carbon dioxide normally being accelerated by a catalyst present in the liquid mixture, and the carbon dioxide gas thus produced assists in formation of a foam.
However, many commercial grades of polyurethane foam require low foam densities and high foam flexibilities, and it is not possible to produce a low density, highly flexible foam using only carbon dioxide as the foam-forming gas. The water-isocyanate reaction which generates the carbon dioxide also produces solid reaction byproducts which harden the foam, so that if a high proportion of water is used in the reaction mixture, the resultant foam is too hard for many applications. Moreover, the water-isocyanate reaction is highly exothermic, to such an extent that if a high proportion of water is included in the reaction mixture, the resultant foam is produced at a temperature sufficiently high that it may be scorched by atmospheric oxidation; indeed, there have been occasional instances where foams produced from reaction mixtures containing high proportions of water have spontaneously combusted. (Incidentally, the exothermic reactions involved in polyurethane foam manufacture continue for a considerable time, typically several hours, after formation of the solid foam, so heat damage to the foam may not always be visible by observing the foam as it passes along the production line.)
Accordingly, it is conventional to include in the foam-forming mixture an auxiliary blowing agent, that is to say a blowing agent other than water, this auxiliary blowing agent being a material which is liquid at room temperature but which is relatively volatile so that it can be volatilized during the foam-forming reaction to form a gas which serves to decrease the density of the polyurethane foam without producing an excessively hard foam. Auxiliary blowing agents also serve to prevent foam becoming too hot, since some of the heat generated by the water is absorbed as latent heat of vaporization in volatilizing the auxiliary blowing agent.
For many years, the preferred auxiliary blowing agents for use in commercial polyurethane production were the chlorofluorocarbons (CFC's), especially trichloromonofluoromethane, sold under the trademark "Freon 11". The chlorofluorocarbons are highly desirable auxiliary blowing agents because of their high volatility (trichloromonofluoromethane has a boiling point of 23.7.degree. C.) and because they are inert and thus do not interfere with the polyurethane-forming reactions. However, the chlorofluorocarbons, which persist for many years in the atmosphere, have recently been discovered to pose a major threat to the environment because they destroy the earth's ozone layer. Consequently, an international agreement has already been signed to greatly reduce annual production of chlorofluorocarbons during the next few years. Thus, it is likely that use of chlorofluorocarbons as auxiliary blowing agents in foam production will be outlawed in the near future.
Methylene chloride has been suggested, and indeed is already in commercial use, as a substitute for chlorofluorocarbon auxiliary blowing agents. Methylene chloride is highly volatile (boiling point 39.8.degree. C.) and inert in polyurethane-forming mixtures. However, methylene chloride is a suspected carcinogen, and has other deleterious effects on workers exposed to the material. Accordingly, the concentration of methylene chloride in the air inside a foam plant must be kept low; the American Conference of Governmental Industrial Hygenists recommends that workers be exposed to not more than 50 ppm. of the chemical, while the Occupational Safety and Health Administration Permissible Exposure Limit is 500 ppm. Keeping the levels of methylene chloride in a foam plant below 50 ppm. may require additional ventilation equipment, with an associated increase in costs. In addition, the chemical is a recognized environmental pollutant, and both the Federal and state governments are beginning to limit releases of the chemical from plants which employ it. Thus, in the near future plants using methylene chloride as an auxiliary blowing agent may be faced with the substantial additional expense of installing scrubbers or similar equipment to remove methylene chloride from air and/or other gases discharged from the plant. In addition, California has recently proposed that emissions of methylene chloride in that state be subject to a heavy "pollution" tax, and other states are likely to follow a similar course of action.
Incidentally, although it might be thought that methylene chloride (molecular weight 85) would be more effective as an auxiliary blowing agent than trichloromonofluoromethane (molecular weight 137.4), in practice it is found that, in similar formulations, the two auxiliary blowing agents have essentially the same blowing activity per unit weight. Thus, the activity per unit weight of an auxiliary blowing agent cannot readily be predicted simply from its molecular weight.
Thus, both the auxiliary blowing agents presently used commercially for the production of polyurethane foams cause significant environmental problems, and it appears likely that the industry will, within the next few years, not be permitted to use these two auxiliary blowing agents at all, be subject to heavy penalties for their use, or will be required to take expensive precautions to prevent their release into the environment. Thus, there is a need for alternative auxiliary blowing agents for use in the production of polyurethane foams.
In theory, the only real requirements for an auxiliary blowing agent are that it be liquid at about ambient temperature (so that it can be introduced into the foam-forming mixture as a liquid), sufficiently volatile that it volatilizes during the formation of the polyurethane foam, and sufficiently inert that it does not interfere with the foam-forming reactions, a wide variety of materials have been proposed for use as auxiliary blowing agents. However, none other than chlorofluorocarbons and methylene chloride appear to have been used industrially, and some of the proposed compounds have disadvantages so severe that they are totally impracticable for use in industrial plants. Thus, for example, U.S. Pat. No. 4,546,122, issued Oct. 8, 1985 to Radovich et al., states concerning auxiliary blowing agents for flexible polyurethane foams:
Suitable blowing agents include water and/or readily volatile inorganic or organic substances. Appropriate organic blowing agents are acetone, ethyl acetate, halogen-substituted alkanes such as methylene chloride, chloroform, ethylidene chloride, vinylidene chloride, monofluorotrichloromethane, chlorodifluoromehhane, dichlorodifluoromethane; and butane, hexane, heptane or diethyl ethers. Inorganic blowing agents which may be used are air, CO.sub.2 and N.sub.2 O. A blowing effect may also be achieved by adding compounds which decompose at the reaction temperature to give off a gas (e.g., nitrogen, given off by azo compounds, such as azodicarbonamide or azobutyronitrile). . . .
Of the compounds other than CFC's and other haloalkanes in this list, diethyl ether is obviously impracticable as an auxiliary blowing agent under industrial conditions because of its flammability and anesthetic effect, while the use of carbon dioxide or nitrogen requires special equipment for injection of the gaseous auxiliary blowing agents into the developing foam, and consequently these gaseous auxiliary blowing agents are not used in practice for low density polyurethane foam.
U.S. Pat. No. 3,121,699 to Merriman describes the preparation of foamed polyurethane materials using a polyalkylene ether polyol having a mean molecular weight of more than 1500, an organic polyisocyanate, water, a foam stabilizing agent and a catalyst, and acetaldehyde as an auxiliary blowing agent, the acetaldehyde being vaporized by the heat produced in the exothermic reaction of the ingredients, the acetaldehyde being included upon the ingredients forming the foam and setting of the foam being effected within a mold.
U.S. Pat. No. 3,165,483 to Gemeinhardt et al., describes the preparation of a skeletal polyurethane foam which is produced using as an auxiliary blowing agent a ketone containing from 5 to 18 carbon atoms. Alternatively, an aldehyde having from 4 to 18 carbon atoms can be used. Among the specific ketones mentioned is methyl n-propyl ketone.
U.S. Pat. No. 3,179,626 to Beitchman describes the preparation of polyurethane and polyisocyanate foams using as co-catalyst a mixture of diazabicyclooctane and an aldehyde containing from 1 to 10 carbons atoms, there being from 0.01 to 10 parts of diazabicyclooctane (sold commercially as DABCO) per part of aldehyde. In this patent, the aldehyde appears to be strictly a co-catalyst; Example V uses a conventional haloalkane as an auxiliary blowing agent.
Klesper, "Application of volatile organic liquids for expanding flexible urethane foam", Rubber Age, October 1958, 84-87, describes experiments in which various organic liquids were tested as auxiliary blowing agents for the production of flexible polyurethane foams by the two stage or "two shot" process, in which an isocyanate is first reacted with a polyol to produce a pre-polymer, and at some later time, this pre-polymer is reacted with water and catalysts to yield the polyurethane foam. This article states that acetone was not very effective in the blowing action because of a too high boiling point and high solubility in the finished polymer, which also causes the acetone to leave the finished foam slowly. The article further states that pentane alters the surface properties of the foam during rise.
U.S. Pat. No. 3,586,651 to Salyer et al. describes a process for the preparation of closed-cell rigid polyurethane foams from polyether polyols and polyisocyanates using, as a "pneumatogen" (i.e., an auxiliary blowing agent), a cyclopentane/acetone mixture. This patent teaches away from the use of acetone alone, citing the aforementioned Rubber Age article for the proposition that single compounds such as acetone or pentane are not found satisfactory, since acetone has too high a boiling point and is too soluble in the finished polymer, whereas pentane gives coarse, uneven cell structure, and cyclopentane alone cannot be used because of its immiscibility with prepolymers and liquid resins.
U.S. Pat. No. 3,193,525 to Kallert et al. describes a process for the preparation of polyurethane polymers which comprises reacting an organic polyisocyanate with a hydroxyl polyester in the presence of a carbodiimide and an enolizable compound in which from 20-95% of the compound is in the keto form, the organic compound being free of carboxylic acid groups. Among the preferred enolizable organic compounds is acetylacetone; acetone itself is not mentioned, presumably because it is not sufficiently enolizable.
U.S. Pat. No. 3,661,885 to Haddick et al. describes the use, as catalyst for polyurethane production, of complexes of stannous salts with organic complexing agents. The organic complexing agent made be a ketone, lactone or amine. The patent states (column 1, lines 48-50) that "suitable ketone includes a dialkyl ketone, e.g. methyl lower alkyl ketone such as methyl isobutyl ketone)."
U.S. Pat. No. 3,985,688 to Speech describes a method of sealing a water-bearing structure, for example a sewer, by placing at the point of leakage a fluid sealing composition comprising a water-miscible mixture of isocyanate-terminated polyoxyethylene urethane prepolymers in a compatible, water-miscible solvent, and curing the prepolymer by reaction with water to form a seal. The water-miscible solvent can be acetone.
U.S. Pat. No. 4,055,522 to Ashida et al. describes a process for producing an isocyanate-based polymer by catalytically reacting a polyisocyanate alone or in combination with a polyhydroxy compound, with the addition of an aromatic aldehyde having no functional group capable of reacting with the isocyanate group, as a smoke suppressing agent.
U.S. Pat. No. 4,251,635 to Stone describes a flexible polyurethane foam having a reduced tendency to form burning embers when it is ignited and burned; to this end a ketone or benzaldehyde, and optionally a flame retardant, is incorporated into the mixture used to produce the foam.
U.S. Pat. No. 4,259,454 to Crivello describes curable organic resin compositions, such as epoxy resins (polyurethanes are not mentioned). In column 8 of this reference does contain a discussion of volatile organic solvents which can be used to produce rigid or flexible foams, and the list given includes acetone.
U.S. Pat. No. 4,263,408 to Meyborg et al. (assigned to Bayer AG) describes a process for the production of molded foams having a compact surface and cellular core, these foams being formed by reacting a polyisocyanate, a relatively high molecular and/or low molecular weight compound having an average of at least two isocyanate-reactive hydrogen atoms, a catalyst and an auxiliary blowing agent. This patent contains a list of potential auxiliary blowing agents essentially identical to that in U.S. Pat. No. 4,546,122 discussed above.
U.S. Pat. No. 4,303,758 to Gusmer describes a closed cell phenol-aldehyde foam (there is no discussion of polyurethane foams) which is produced using an auxiliary blowing agent. Auxiliary blowing agents are discussed in detail in the paragraph bridging columns 6 and 7 of the reference and acetone is mentioned as a ketone blowing agent.
U.S. Pat. No. 4,401,769 to Malwitz describes a foam made from the reaction of a multifunctional isocyanate and a catalytic amount of an isocyanate polymerizing catalyst. In the production of the foam a softening agent is used, and this softening agent can be acetone.
The applicant has conducted experiments to determine whether acetone can be used as a substitute for CFC's and methylene chloride in the production of polyurethane foams. The results of these experiments have generally confirmed to statements in the aforementioned Rubber Age article that acetone has too high a boiling point to act satisfactorily in most conventional processes for the production of polyurethane foams. However, applicant has discovered that, by careful control of the process employed, high quality foams can be obtained using acetone as the auxiliary blowing agent.